Detection of UH3+ and ThH3+ molecules and 236U background studies with low-energy AMS

The performance of the TANDY AMS facility (0.6 MV) at ETH Zurich has been significantly improved for actinides with the installation of an additional 130 degrees magnet on the high-energy side. The background originating from neighboring masses is reduced by three orders of magnitude in the new setup, leading to an abundance sensitivity of 10(-10). This now allows U-236/U-238 measurements down to the 10(-12) level. Systematic studies of the possible background for U-236(3+) at the compact TANDY system show that actinide-hydride molecules in charge state 3+ do exist. ((ThH)-Th-232-H-1)(3+), ((UH)-U-235-H-1)(3+) and ((UH)-U-238-H-1)(3+) can cause a severe background at masses 233, 236, and 239, respectively. The existence of the ((ThH)-Th-232-H-1)(3+) molecule was verified unambiguously by analyzing its breakup. The actinide-hydride molecules can be destroyed efficiently using higher stripper pressures. At an ion energy of 1.2 MeV an overall efficiency of more than 10(-4) can be achieved using a gas ionization detector. The high efficiency in combination with the low background opens the spectrum of possible applications to both anthropogenic (U-236/U-238 = 10(-7) to 10(-9)) and natural samples (U-236/U-238 < 10(-10)). (C) 2012 Elsevier B.V. All rights reserved.